The present invention relates to a method for controlling arc welding and ana apparatus therefor, and more particularly to a method and an apparatus in which the temperature of the welded portion on the surface of a workpiece is monitored in order to accomplish high quality welding.
In general, arc welding is used for bonding two pieces of plates together. When thin plates are to be welded together, too much heat causes them to be melted down, whereas insufficient heat fails to uniformly weld them together up to the reverse sides thereof. If the welding is not effected uniformly, defects develop in the welding portion, or corrosion takes place in the gaps that develop in the poorly welded portions. In order to obtain uniform beads which are homogeneously melted up to the reverse sides, it is necessary to detect at all times the welding conditions, or the depth of penetration of the weld and control the welding apparatus. In welding workpieces whose reverse sides cannot be seen, for example pipes, however, it is required to determine relying upon the information obtained from the front surface whether the welding is effected up to the reverse sides or not. Usually, the welded condition can be best determined by detecting the temperature of the welding portion.
For example, Japanese Publication of Patent No. 44-1824 (1969) describes a welding apparatus in which an arc light of a welding electrode and a molten pool are simultaneously detected by a photocell, and the welding speed is controlled depending upon the output of the photocell. However, the above mentioned apparatus is not capable of properly judging the welding condition under different cooling conditions because the intensity of the arc light varies depending upon the welding current or arcing length, but not depending upon the cooling condition of workpieces. In particular, this tendency appears strikingly for such materials as copper and aluminum which have high heat conductivity and low melting points.
In contrast with the method of monitoring the arc light as well as the radiated waves or radiant-energy emitted from the welding portion, it is well known that a radiant-energy detector monitors the temperature at the welding portion which is remote from the arc-generating portion. For instance, U.S. Pat. No. 3,370,151 issued to Normando on Feb. 20, 1968 discloses a welding apparatus in which a radiation-type thermometer is driven in a direction perpendicular to the welding line on the weld bead at a point which is separated by a predetermined distance from the molten pool. The light emitted from the bead passes through lens and it is reflected at right angles by a reflection mirror provided in the radiation-type thermometer.
In the above apparatus, the temperature distribution of a portion which is already solidified and which is located behind a portion which is now being welded, is measured by the reflection mirror and the photocell. Furthermore, the bead width is detected from the temperature distribution, and the welding current is so controlled that the bead width becomes constant. Such system is capable of correctly measuring the temperature at the surface of the weld bead, but is not capable of measuring the width of the weld bead from the measured temperature distribution. In addition, the discrimination of the molten state is difficult.